The navigational requirements of remotely operated or autonomous underwater vehicles (otherwise known as ROV/AUVs) have been provided through the use of doppler sonar, transponder navigation, and inertial navigation. These navigational techniques have been used for missions of relatively short duration (i.e. one day or less). The doppler sonar systems can have problems with large errors at velocities less than 2 knots. Vehicles assigned to longer term missions tend to develop navigation errors due to the long-term drift of the vehicle's navigation system.
There are applications which call for autonomous or semiautonomous vehicles to operate unsupervised for long periods of time with little or no opportunity for navigational updates. These applications require navigation systems which are more accurate and have lower long-term drift than those presently in use today.
There are also applications which require navigation systems that produce very little or no detectable emanations. Doppler sonars in use today produce high levels of detectable acoustic energy, making their use in such situations undesirable.
Correlation techniques were originally invented and successfully adapted in the radar industry. General Electric has successfully adapted them to sonar technology with their Correlation Velocity Log device. The G.E. Correlation Velocity Log is an instrument which measures the speed of a ship or underwater vehicle relative to the ocean bottom. Velocity measurements are obtained from space/time correlation measurements. In the case of the G.E. system an acoustic signal is echoed from the ocean floor and the returning echo signature is then captured with an array of hydrophones. Thus, an acoustic snapshot is captured for the vehicle's position. Successive echoes are received and each received signature is correlated with the previous one. Distance traveled can then be extracted from the position data. These systems are currently in use on ocean going vehicles.
Primitive optical correlation techniques were also briefly pursued by the early missile industry for terrain navigation but the required computational power made it impractical as a navigation aid at that time. Computer technology advances and improved optical sensor resolution now make the application of optical correlation techniques both practical and desirable as an aid to a navigational system.